C840 up vars

[BSick7]

Added --var to nullstone up allowing users to specify Terraform variable values upon launch.

This is especially helpful for the quickstarts where a service_port is different:

nullstone up --app=app --env=dev --var=service_port=8000

This --var performs light validation of the input data. Here is an example of invalid input:

$ nullstone up --block=test --env=dev -var=service_count=fail -var=service_cpu=x
Performing workspace command (Org=demo, Block=test, Stack=, Env=dev)

"--var" flags contain invalid values:
    * service_count: expected 'number' - strconv.ParseFloat: parsing "fail": invalid syntax
    * service_cpu: expected 'number' - strconv.ParseFloat: parsing "x": invalid syntax